1. Field Of The Invention
The present invention relates to a procedure for the control of solid-state switches in three-phase frequency converter and rectifier/inverter bridges, said control employing pulse width modulation. The present invention also relates to a modulator unit designed for implementing that procedure.
2. Description Of Related Prior Art
A symmetric modulation procedure for the control of frequency converter and rectifier bridges is presented in IEEE Transactions on Industry Applications, Vol. IA-21, No. 5, Sep./Oct., 1985, in an article entitled "PWM control Techniques for Rectifier Filter Minimization", pp. 1206-1214, written by Phoivas D. Ziogas, Young-Goo Kang and Victor R. Stefanovic. The principle of said modulation procedure is based on the comparison of a non-continuous carrier wave of a given frequency and a sinusoidal modulation reference signal. During the middle 60.degree. interval of each half-cycle of the sine wave, the modulation pulses are generated by applying a reference consisting of the sum of the modulation references for the other two symmetrical phases. The procedure of Ziogas et al only allows certain relationships between the frequency of the carrier and that of the fundamental wave (n=k+3, k=1,2,3, . . . ), and the modulation carrier needs to be synchronized with the modulation reference. For these reasons, the procedure of Ziogas et al is difficult to realize without modifications, especially in the low frequency region, and in particular if the carrier frequency is to be kept constant or within definite limits.